Titanium dioxide is well known for use as a pigment and white opacifying agent. For example, titanium dioxide pigments are used in connection with coating formulations including paint and ink formulations, paper compositions, polymer compositions and other products. Titanium dioxide pigments are typically produced in powder form by either the sulfate process or the chloride process.
In the sulfate process for producing titanium dioxide, a titanium slag ore is dissolved in sulfuric acid to form titanyl sulfate. The titanyl sulfate is then hydrolyzed to form hydrous titanium dioxide. The hydrated titanium dioxide is heated in a calciner to grow titanium dioxide crystals to pigmentary dimensions.
In the chloride process for producing titanium dioxide, a dry titanium dioxide ore is fed into a chlorinator together with coke and chlorine to produce a gaseous titanium halide such as titanium tetrachloride. The produced titanium halide is purified and oxidized in a specially designed reactor at a high temperature to produce titanium dioxide particles having a desired particle size. Aluminum chloride is typically added to the titanium halide in the oxidation reactor to facilitate rutile formation and control particle size. The titanium dioxide and gaseous reaction products are then cooled and the titanium dioxide particles are recovered.
Whether produced by the sulfate process or the chloride process, the produced titanium dioxide particles are typically coated with one or more hydrous metal oxide inorganic materials to modify the properties and characteristics of the pigment for particular applications. For example, the pigment particles are often coated with compounds that improve the opacity, light stability and durability of the pigment. Examples of hydrous metal oxide inorganic materials used to coat titanium dioxide pigments are alumina and silica.
A primary property that a titanium dioxide pigment contributes to paint, paper, plastic and other products is hiding power. The hiding power of a titanium dioxide pigment is based on the ability of the pigment to scatter light in the base product (for example, a paint formulation) to which it is added. The ability of a titanium dioxide pigment to scatter light in the base product to which it is added (the “light scattering efficiency”) depends on various factors, including the particle size of the pigment and the difference in refractive index of the pigment particles and their surroundings. A large difference in the refractive index of the pigment particles and the base product results in high scattering efficiency. The light scattering efficiency of the pigment further depends, for example, on the proximity of the pigment particles to one another. These factors have been addressed in various ways with varying degrees of success.
A problem associated with the use of titanium dioxide in aqueous based coating formulations such as paint and ink formulations is the tendency of the titanium dioxide particles to agglomerate in the coating formulations. Agglomeration of the titanium dioxide particles in an aqueous based coating formulation can adversely impact desirable properties of the pigment including the opacity, brightness, tint strength and other optical properties of the pigment.
For example, problematic pigment agglomeration in aqueous based coating formulations often occurs while the paint film dries after the paint film has been applied to a substrate (for example, a wall). Sometimes referred to as optical crowding, this phenomenon can decrease the light scattering efficiency of the pigment particles. Consequently, the tint strength of the pigment can be diminished.
Agglomeration of pigment particles in an aqueous media is increased when the pigment is utilized in a coating formulation at a relatively high pigment volume concentration. When the volume concentration of the pigment in the coating formulation increases to a certain level, the light scattering efficiency of the pigment can substantially decrease. At high volume concentrations, the pigment particles are closer to one another, resulting in an overlap of the respective light scattering cross-sections of the particles. As a result, the light scattering efficiency of the dispersed pigment is decreased. Additionally, the optical crowding effect can also decrease the light stability, brightness and opacity of the pigment.
Various techniques have been used to diminish optical crowding and address the other problems noted above. For example, fillers and extenders such as clay, calcium carbonate, alumina and silica have been added to aqueous based coating formulations to space adjacent pigment particles apart from one another. Hollow sphere, opaque polymers have been added to aqueous based coating formulations to create air voids in the formulations that function to space the pigment particles apart. Also, pigment particles have been coated with certain inorganic compounds that modify the surface properties of the particles to discourage the particles from agglomerating. Although such techniques have been utilized with varying degrees of success, room for improvement remains.